1) Field of the Invention
The present invention is a method for increasing the bandwidth of a parametric sonar system by using multiple primary frequencies rather than only two primary frequencies.
2) Description of Prior Art
Parametric sonar generates narrow beams at low frequencies by projecting sound at two distinct primary frequencies with high amplitudes. Nonlinear acoustic interaction of sound at the two primary frequencies generates sound at a difference frequency. For example: if sound is transmitted at 80 kHz and 83 kHz, there will also be sound at 3 kHz (the difference frequency). Sound is also generated at a “sum” frequency, but this usually is not useful since acoustic energy at that frequency attenuates faster than at the primary frequencies.
While parametric sonar can be useful for many applications, there are also drawbacks. For example: only the energy associated with the nonlinear interaction at the difference frequency is actually used. Acoustic energy of the primary frequencies such as at 80 kHz and 83 kHz attenuates quickly. Acoustic energy at the nonlinear interaction such as at 3 kHz attenuates slowly.
This energy resulting from the nonlinear interaction can be used for the sonar functions of detection, classification, localization and tracking. A narrower beam from the sonar reduces reverberation because the area insonified around a target is smaller with the result of less energy scattered back from the surrounding water. Scattered back energy typically represents three percent of the total acoustic energy projected.
For example, if an attempt is made to detect a target at a range of 1 kilometer with a beam width of four degrees; then the beam will insonify the target within a circle radius of 70 meters (note that the circle will be perpendicular to the direction of the beam). As a result, scattered energy will be received from the target (which is desired), but scattered energy will also be received from biologics and other small targets within the 70 meter circle (which is not desired). Biologics and other small targets are not wanted because they can obscure a signal.
If the beam width can be reduced to two degrees; the circle radius becomes 35 meters. However, the same scattered energy is received from the target, but less from the other volumetric targets within the circle. Parametric sonar provides a way to obtain narrower beams.
Relevant prior art references include Konrad (U.S. Pat. No. 3,964,013) which discloses that an output frequency range of a speech generator was between 0.5 and 2.5 kHz. The output frequency of oscillator was 22 kHz, giving rise to the frequencies of the signals in the output of a modulator to be 19.5 to 21.5 and 22.5 to 24.5 kHz. These signals cause cavitation adjacent or at the face of the transducer which in turn allows the difference frequency range (i.e., frequency range between 2.5 kHz and 4.5 kHz to be generated).
Huckabay et. al. (U.S. Pat. No. 4,320,474) discloses an array of parametric sources. A source is shown as consisting of two primary sources, each of which is a planar circular transducer of radius (d/2) driven at or slightly below saturation at primary frequencies F1 and F2 by suitable means shown as a frequency source and amplifier. Saturation limited primary sources are operated in a kd region selected to provide a moderately directive radiation field, where k is the average wave number of the primary frequencies.
As shown in the reference, the primary source launches in a medium with dual frequency primary beams collimated in the near field and spherically spreading in the far field. The primary frequencies launched in the primary beams of each directional primary source interact and parametrically generate two difference frequency beams as the primary beams propagate. The difference frequency beams interfere and the coexistent primary radiations parametrically interact in a crosshatched common region; producing a resultant difference frequency beam.
Lagier (U.S. Pat. No. 4,712,201) discloses a schematic of a parametric sonar and the processing of the signals for control of the torpedo rudders. The signals received by transducers are processed by reception assemblies. Item no. 60 of the reference is a decision circuit using signals at the primary and secondary frequencies and item no. 70 represents the torpedo direction and dive rudder controls.
Piloting of the vehicle is done in a known manner with the phase of the signals received by the transducers used to produce reception channels at the primary frequencies and other transducers arranged for this purpose at a vertical distance. For example, an oscillator transmits on ten frequencies, three times in succession. The reason for this sequential transmission is that the high frequency transducer power capacity is limited and does not allow transmission on all the frequencies in every direction simultaneously at the acoustic power level required for a parametric output.
The Lagier reference allows multiple frequencies to be transmitted simultaneously to form multiple parametric beams. However, the multiple nonlinear interactions are not taken advantage of in order to generate additional efficiencies, bandwidth, and SNR. This can be seen most clearly in paragraph 2, lines 15-24 of the reference. The primary high frequencies are denoted F and Fi, where i represents a plurality of frequencies. The difference frequencies generated are listed as f1=F−F1, f2=F−F2, up to fq=F−Fq.
Also, the Lagier reference does not teach or imply the use of difference frequencies such as f12=F1−F2, f23=F2−F3, etc., which involve multiple nonlinear interactions. Instead, each beam is steered in a different direction. The increased efficiency, bandwidth, and SNR arising from the use of multiple nonlinear interactions is not realized.
Ruffa (U.S. Pat. No. 6,704,247) provides first and second signal transducers in communication with a liquid environment. The first transducer is joined to a first amplifier and the second transducer is joined to a second amplifier. The amplifiers are joined to first and second oscillators. The second oscillator is capable of generating a signal at a second frequency. The transducers are oriented so that transmitted acoustic waves overlap in an overlap region.
Giannakis et. al. (U.S. Pat. No. 7,561,613) discloses a transmitter and includes a user code generator that generates digital carrier multi-band user codes for a baseband UWB signal of a user in UWB system. Different from orthogonal frequency division multiple access (OFDMA) in narrowband systems; the baseband SC and MC-UWB spreading codes described herein are real. UWB signaling with either the SC or the MC spreading codes occupies multiple frequency bands and the resulting multi-band transmission has multipath diversity gains.
Rees (United States Patent Publication No. 2004/0174770) illustrates a multiple-projector array embodiment. In such an embodiment, it is possible to apply the transmitter-side adaptive equalization of the Guass-Rees primary waveform as a feedback corrected amplitude and phase adjustment on a per frequency bin basis due to the sub-division of this waveform into multiple, contiguous but non-overlapping frequency bins in filters. This application has gone abandoned.
Rees (United States Patent Publication No. 2007/0274152) discloses an embodiment of a transmitter. The variant of the transmitter has suitable components in place of a computer that is also imbedded into the transmitter, etc. redesignated as a transmitter 2B. A multiple-projector array embodiment is illustrated. This application has gone abandoned.
Cushchieri et. al. (United States Patent Publication No. 2012/0186422) discloses a transducer-array assembly. The assembly may include one of more separate arrays and is operable to generate various acoustic waves, such as low-frequency broadband (LFBB) sonar waves. For example: the assembly may include a transmit array and a receive array. In an embodiment, the transmit array may be approximately three feet long by twenty inches high, with an active area of approximately twelve inches long by eight inches wide and the receive array may be approximately four feet long by twenty inches high. And both the transmit and receive arrays may include multiple transducer elements that the controller may independently process, or otherwise control, to shape the transmitted and received acoustic waves into beams, and to control the directions of these beams.
Based on the teachings of the prior art references, it evident that there is a continuing need for increased efficiency by the use of multiple frequencies. Although, the very low efficiency of traditional parametric sonar has always severely limited the applicability of the sonar; it feasible that this limitation can be overcome.